Since the radar level gauging was developed as a commercial product in the 1970's and 1980's, frequency modulated continuous wave (FMCW) has been the dominating measuring principle for high accuracy applications. An FMCW measurement comprises transmitting into the tank a signal which is swept over a frequency range in the order of a few GHz. For example, the signal can be in the range 25-27 GHz, or 9-10.5 GHz. The transmitted signal is reflected by the surface of the contents in the tank (or by any other impedance transition) and an echo signal, which has been delayed a certain time, is returned to the gauge. The echo signal is mixed with the transmitted signal to generate a mixer signal, having a frequency equal to the frequency change of the transmitted signal that has taken place during the time delay. If a linear sweep is used, this difference frequency, also referred to as an intermediate frequency (IF), is proportional to the distance to the reflecting surface. The mixer signal is often referred to as an IF signal.
More recently, the FMCW principle has been improved, and today typically involves transmitting not a continuous sweep but a signal with stepped frequency with practically constant amplitude. When the transmitted and received signals are mixed, each frequency step will provide one constant piece of a piecewise constant IF signal, thus providing one “sample” of the IF signal. In order to determine the frequency of the piecewise constant IF signal, a number of frequencies, N, greater than a number stipulated by the sampling theorem will be required. The distance to the reflecting surface is then determined using the frequency of the IF signal in a similar way as in a conventional FMCW system. Typical values can be 200-300 IF periods at 30 m distance divided in 1000-1500 steps.
It is noted that also a continuous IF signal, resulting from a continuous frequency sweep, may be sampled in order to allow digital processing.
Although highly accurate, conventional FMCW systems (continuous as well as stepped) are relatively power hungry, making them less suitable for applications where power is limited. Examples of such applications include field devices powered by a two-wire interface, such as a 4-20 mA loop, and wireless devices powered by an internal power source (e.g. a battery or a solar cell).
The main power consumer is the microwave module, which, due to the requirements on frequency accuracy, requires relatively high power to generate and emit the microwave energy during each sweep. Between sweeps suitable means can be used to store power, so that a lower average power can used to power the microwave module for the duration of the sweep. However, due to space limitations and intrinsic safety (IS) requirements, such power storage capacity is severely limited. Therefore, it is crucial to limit the active period of the microwave module, i.e. to limit the duration of the sweep. Further, it is necessary to limit the sampling rate, in order to reduce the power consumption in the analogue signal processing and the A/D conversion. Finally, from a performance point of view, it is advantageous to have a wide bandwidth, providing a high resolution (i.e. high accuracy).
For any sampled FMCW system (continuous sweep or stepped), the maximum measuring distance (range), L, is determined as L=Nc/4B, where N is the number of samples, c is the speed of light, and B is the sweep bandwidth. In case of a stepped frequency sweep, N will typically correspond to the number of different frequencies used. The sweep time, T, is T=N/fs, where fs is the sampling rate of the AID conversion. In case of a stepped frequency sweep, fs will typically also be the stepping rate of the sweep.
From these simple relationships, it is clear that an increased bandwidth B will lead to a reduced range L unless the number of samples N is increased. However, as the sampling frequency is fixed at a reasonable value from an A/D conversion standpoint, any increase of the number of samples will inevitably lead to an increased sweep time.
For a given range, there is thus a tradeoff between accuracy (bandwidth) on one side, and power consumption (sweep time) on the other. This trade-off is present in any sampled FMCW system, in cases with a continuous sweep as well as with a stepped frequency sweep.